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Thursday, July 23, 2020

July 23, 2020
Credit...Flórián Tóth

At first glance, American paddlefish and Russian sturgeon seem about as different as two fish can be.

The Russian sturgeon, whose eggs are used to make top-shelf caviar, is a carnivore that hoovers crustaceans and smaller fish off the floor of rivers, lakes and coastal areas the world over. The American paddlefish, found in only 22 of the United States, is a filter feeder that strains zooplankton from the water. It has a comically long snout covered with tens of thousands of sensory receptors.

Yet somehow, when sperm from an American paddlefish and eggs from a Russian sturgeon were combined in a lab, life found a way and a hybrid of the two species was born.

“I did a double-take when I saw it,” said Solomon David, an aquatic ecologist at Nicholls State University in Louisiana. “I just didn’t believe it. I thought, hybridization between sturgeon and paddlefish? There’s no way.”

The Hungarian researchers who accidentally created this hybrid reported what they had done in May in a study published in the journal Genes. The internet has already nicknamed the hybrid the sturddlefish. The Hungarian team’s research highlights how creatures that seem distant on the tree of life may be sitting on closer branches than expected.

Sturgeons and paddlefish are among the largest, longest-living, slowest-growing freshwater fish on Earth. They are also among the most endangered. The paddlefish is the only such species that remains in existence after a Chinese species was declared extinct, and sturgeon are “more critically endangered than any other group of species,” according to the International Union for the Conservation of Nature.

Credit...Kaldy et al., Genes 2020

Habitat loss, overfishing and pollution have taken a heavy toll on paddlefish and sturgeon over the last century, which is why Attila Mozsár, a senior research fellow at the Research Institute for Fisheries and Aquaculture in Hungary and a co-author of the study, and others have been trying to breed both fish in captivity.

Last year, researchers were trying to induce gynogenesis, a form of asexual production that requires the presence of sperm, but not the actual contribution of their DNA, in Russian sturgeon.

Something unexpected happened: The paddlefish sperm the researchers were using successfully fertilized the sturgeon eggs.

“We never wanted to play around with hybridization. It was absolutely unintentional,” said Dr. Mozsár.

Hundreds of hybrids emerged from those eggs and a month later, more than two-thirds of them were still alive. Around 100 of these hybrids are alive today.

Both creatures are known as “fossil fish” because of their ancient lineage. Their last common ancestor swam during the age of the dinosaurs, and the two have been evolving independently, on opposite sides of the planet, for over 184 million years — which makes them nearly twice as evolutionarily diverged as humans and mice. That led scientists to assume that they were too evolutionarily diverged to be hybridized.

The sturddlefish created in Hungary exhibited traits from both species. Although all have their mother’s mouth and carnivorous appetite, some have their father’s fins and snout — though slightly smaller. After conducting DNA analysis on eight sturddlefish, the researchers were able to separate the hybrids into two groups. Some of the sturddlefish got a double-dose of DNA from their mother, and look far more like sturgeon than paddlefish. The others received nearly equal amounts of maternal and paternal DNA, and look like a perfect mash-up of the two species.

Russian sturgeon and American paddlefish may not seem as if they have much in common, but the fact that they can hybridize suggests they may be more alike than previously thought.

“Many aspects of their anatomy and physiology are actually very similar,” said Dennis Scarnecchia, professor of fishery resources at the University of Idaho. Both species have scaleless skin, spiral valve intestines and cartilaginous endoskeletons.

Although it seems unlikely that two species would have so much in common after spending ages evolving independently, 184 million years isn’t actually that long for these fish.

“These living fossil fishes have extremely slow evolutionary rates, so what might seem like a long time to us isn’t quite as long of a time to them,” Dr. David said.

Although Dr. Mozsár and his colleagues plan to continue caring for the hybrids they created, they have no plans to make more. The researchers suspect that sturddlefish, like ligers, mules and so many other human-made hybrids, are sterile, so they have no value for caviar production. The creation of more sturddlefish could also imperil wild fish populations.

Nevertheless, the sturddlefish has captured the imaginations of scientists around the world.

“I think it’s pretty cool that these living fossils can still surprise us,” Dr. David said.

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